At present, analytical laboratories are faced with the annoying contrast between long sample preparation times and very rapid analysis of the samples. State of the art analytical instruments must wait, and often sit idle for long periods of time, for samples to be prepared by methods using old technologies. The productivity of analytical laboratories can be improved either by increasing the performance of analytical instruments or by reducing sample preparation times. Therefore, sample preparation time has become the rate-limiting factor to higher productivity in modern laboratories.
A classic sample preparation technique requires that one or more components of a sample is selectively solubilized and separated from bulk samples prior to analysis. The time required to perform such traditional separations, known as extractions, is a function of the type of solvent and the solubility of the species to be extracted at the extraction temperature.
In an open vessel, the highest extraction temperature is the boiling point of the solvent system, at atmospheric pressure. When extractions are performed in a closed vessel system, it is possible to increase the extraction temperature. As is well known, an increase in pressure of a closed vessel system results in an increase of the boiling point of the solvent system. Increasing the extraction temperature can have the effect of dramatically increasing the solubility of the extractant species, thereby increasing the efficiency of the extraction process and reducing the time needed for the extraction process.
Microwave induced heating of solvent extractions for analytical sample preparations has been known since the mid-nineteen eighties. Ganzler et al., described "A new method for the extraction and high-performance liquid chromatographic determination of vicine and convicine in fababeans", Chromatography, the State of the Art, Akademiai Kiado, Budapest, (1985) pp. 435-442, which was presented at the 2nd International Eastern European-American Symposium of Chromatography in 1984. In 1986, Ganzler et al., also reported microwave extraction as a novel sample preparation method for chromatography in the Journal of Chromatography, 1, pp. 299-306. Further reports by Ganzler and Salgo for the use of microwave energy to heat an extractant medium are found in Z. Lebensm. Unters. Forsch., 184, pp. 274-276, 1987. However, most of the microwave energy is absorbed by the extracting solvent resulting in heating of the extracting solvent but very little energy reaches the inner parts of the material to be extracted.
In U.S. Pat. No. 5,002,784, ParJ teaches that biological materials containing microwave absorbing substances, which are subjected to microwave radiation while in contact with a microwave transparent or partially transparent extracting solvent, result in differential heating of the material to be extracted. ParJ also discloses in U.S. Pat. No. 5,732,476 a method for microwave-assisted separation using volatiles, which are carried out in a flow-through system having a microwave absorbing component.
Thus far the microwave heating has relied on the presence of polar, microwave absorbing solvents and on mixtures of polar and non-polar solvents. The recoveries achieved with the use of microwave-assisted extraction processes are comparable or superior to traditional soxhlet extractions but the extraction rates are significantly faster.
A new Milestone, Inc..RTM. technique allows microwave extractions using polar as well as non-polar solvents, while agitating the sample and solvent to achieve mixing and optimum recoveries. Pure, non-polar solvents can be heated by using Weflon.RTM. coated magnetic stir bars that absorb microwave energy and, in turn, heat non-polar, microwave-transparent solvents. Thus, microwave extractions can be performed without the addition of polar co-solvents.
In view of the fact that sample preparation time has become the rate-limiting factor preventing higher productivity in an analytical laboratory, it is an objective of this invention to provide a vessel and a system, which assures fast operation. This is achieved by employing a vessel and a system that is usable for a plurality of chemical or physical processes, such as drying, extraction, filtration, as well as for solvent evaporation and recovery without the need of transferring the sample to another vessel. The fact that a multitude of reactions and processes can be performed in the same vessel and with the same system provides for cost effective and fast operation. Another object of this invention is to provide a vessel and a system facilitating even heating of the sample to avoid so called "hot spots".